I'm hoping someone can help me understand a couple aspects of the attached circuit. It's for a simple, low end MIG welder rated at about 120 amps at 20vdc at 20% duty cycle. Even though it's a small machine, the basic design features are similar to much larger transformer-rectifier MIG's, but without all the bells and whistles to complicate the diagram.
BTW: VR1 is a 10 joule 68vdc varistor, R2 is a 20w 50 ohm wirewound and C1 is 46,000 uf 35vdc.
1) First of all, what's the purpose of (what I'm sure are) ceramic capacitors C2, C3 across the diodes of the center-tapped FWB? I understand these might have some role in supressing noise coming off the diodes--but what difference would that make in this simple machine? There are no IC's anywhere, so what's to matter? Anyway, some manufacturers use them, and some don't. I've seen them in 4-diode FWB's too. What would be a good size for these? .01uf, .1uf, 1uf, ... all of the above. I haven't been able to find a circuit diagram that calls out a value.
2) Some manufacturers also include ceramic capacitors from the output terminals (labeled work and torch in this diagram) to ground. What would these capacitors be doing. What would be reasonable values?
3) What's the purpose of varistor VR1? These are kind of like diacs, right? At a threshold voltage they switch full on--so is this varistor to protect the big capacitor C!, or to protect the fellow welding from high voltage? A varistor in this location doesn't seem to be a common feature in MIG circuits I've seen.
4) I do hate to unambiguously identify myself as ignorant, but what's the purpose of the big resistor R2 across the output? Maybe to limit current in the event of a dead short of the electrode to the work? Not every manufacturer uses them, but I've seen several diagrams with such resistors. On a big 250 amp machine, this resistor might be 300w 50 ohm, if it was present.
Thanks in advance; I really appreciate the help.
BTW: VR1 is a 10 joule 68vdc varistor, R2 is a 20w 50 ohm wirewound and C1 is 46,000 uf 35vdc.
1) First of all, what's the purpose of (what I'm sure are) ceramic capacitors C2, C3 across the diodes of the center-tapped FWB? I understand these might have some role in supressing noise coming off the diodes--but what difference would that make in this simple machine? There are no IC's anywhere, so what's to matter? Anyway, some manufacturers use them, and some don't. I've seen them in 4-diode FWB's too. What would be a good size for these? .01uf, .1uf, 1uf, ... all of the above. I haven't been able to find a circuit diagram that calls out a value.
2) Some manufacturers also include ceramic capacitors from the output terminals (labeled work and torch in this diagram) to ground. What would these capacitors be doing. What would be reasonable values?
3) What's the purpose of varistor VR1? These are kind of like diacs, right? At a threshold voltage they switch full on--so is this varistor to protect the big capacitor C!, or to protect the fellow welding from high voltage? A varistor in this location doesn't seem to be a common feature in MIG circuits I've seen.
4) I do hate to unambiguously identify myself as ignorant, but what's the purpose of the big resistor R2 across the output? Maybe to limit current in the event of a dead short of the electrode to the work? Not every manufacturer uses them, but I've seen several diagrams with such resistors. On a big 250 amp machine, this resistor might be 300w 50 ohm, if it was present.
Thanks in advance; I really appreciate the help.
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